Natural gas reservoirs frequently contain carbon dioxide. Some reservoirs around the world may contain up to about 90% carbon dioxide. Generally, this carbon dioxide must be separated from the natural gas stream because it contains no thermal heating value and it does have acidic properties which can corrode natural gas equipment.
There are numerous methods available for removing carbon dioxide from natural gas streams. Most commonly used is a chemical absorption process wherein a chemical solvent such as ethanolamine is used to selectively and chemically bind the carbon dioxide for removal. This occurs by passing the unpurified natural gas stream through a packed or trayed tower in which the chemical solvent is flowing countercurrently to the natural gas stream.
The chemical solvent will bind and remove the carbon dioxide and a purified natural gas stream emerges from the top of the tower. The chemical solvent then rich in carbon dioxide must be regenerated so that the solvent can be reused or recycled. This typically occurs by lowering the pressure and raising the temperature of the carbon dioxide rich solvent stream in a regeneration tower. This tower generally consists of a reboiler and a reflux condensing system. Heat is added in the reboiler to break the carbon dioxide-solvent bonds and the reflux system condenses any solvent vapors that might escape with the carbon dioxide gas. The purified carbon dioxide emerges from the top of the tower at a pressure substantially lower than the original natural gas stream.
There are two problems with this type of separation system. First, the regeneration process requires considerable energy for heating. There are physical absorption processes available which require less heating for regeneration, however, these process are generally less selective.
Second, the carbon dioxide that emerges from the regeneration tower is at a low pressure. The low pressure carbon dioxide coming from the conventional regenerator often requires additional costs and equipment for recompression in order to inject the carbon dioxide back into the reservoir; a requirement that has become more important due to environmental concerns over "green house" gases such as carbon dioxide. Traditional venting of carbon dioxide in significant amounts to the atmosphere is no longer environmentally acceptable in some parts of the world.
It would be desirable to have a process which effectively separates carbon dioxide from natural gas but requires lower energy consumption and does not significantly lower the pressure on the carbon dioxide removed from the gas stream in the solvent regeneration step.
Another naturally occurring phenomenon associated with natural gas systems is the formation of gas hydrates. Gas hydrates are solids which are formed by a physical reaction between water and low molecular gases such as methane and carbon dioxide. On a molecular level, gas hydrates appear to resemble a class of compounds known as clatharates. Water molecules assemble into cage-like structures with cavities of a specific size trapping hydrate forming gases inside.
A significant amount of experimentation has been done on hydrate formation to address the problems caused by solids buildup in pipelines and processing equipment, particularly in colder environments. It has been determined that hydrate forming compounds have different hydrate forming characteristics. The catastrophic point, that point at which solid hydrate formation begins, is a function of temperature, pressure, and gas composition. This variation in hydrate formation characteristics makes possible a separation process which could selectively remove hydrate forming compounds. However, because gas hydrates are solids, process equipment would have to be able to deal with multi-phase fluids and the problems associated with solids accumulation.
Prior studies of hydrate formation in pure component gases have shown that a significant amount of pre-solid hydrate structures exist in water even when the temperature is several degrees above the point where solid hydrates normally form. These prior pure component gas hydrate experiments have also shown that some gas is retained in the water phase by these pre-solid hydrate formations. The mechanism by which these pre-solid hydrate structures retain gases is not clear. However, hereinafter that mechanism will be referred to as absorption.
Scrubbing natural gas with water to remove some of the impurities contained therein has been known in the art for many years. However, the enhanced absorption which occurs as a result of the formation of pre-solid hydrates and the selectivity at which such absorption occurs has not been known or utilized. The term "selectivity" as used herein means the difference in the amount of one gas absorbed relative to another gas in the same mixture.
It is an object of this invention to provide a separation process which utilizes differences in gas absorption caused by pre-solid hydrate formation characteristics. More specifically, it is an object of this invention to utilize selectivity differences in pre-solid hydrate absorption between carbon dioxide and light hydrocarbon gases to provide a separation system. The term "light hydrocarbon" as used herein generally includes, but is not limited to, any one or more of methane, ethane, propane, and/or natural gas. "Natural gas" is a mixture of two or more of such hydrocarbon gases. Further, it is an object of this invention to provide such a separation system that does not significantly reduce the carbon dioxide pressure or require large energy inputs for regeneration.